Volatile semiconductor memories, such as wellknown random access memories (RAM) are common devices, commonly used in computers and many other digital systems. More and more computers are provided with a cryptographic chip. Such chip may be provided with sensitive information that has to be destroyed upon tampering immediately. In most cases the information that has to be destroyed comprises keys that may be used to decode coded messages. To realize a high system-safety such keys are commonly stored in a volatile semiconductor memory part of a cryptographic chip. In many cases such a key is also generated in the cryptographic chip itself. Since the key does not have to leave the cryptographic chip it is virtually impossible for a hacker to obtain control of the key via a software attack. A hacker in principle can only reach the hard disc of a host computer and not embedded volatile semiconductor memory of a cryptographic chip present in a computer. However a physical attack on the chip is still possible. By opening the chip and probing the volatile semiconductor memory it could be possible to read out information from that memory. An example may be using electron microscopy to determine charge present in memory-cells of the volatile semiconductor memory and thereby the digital information contents of that memory.
U.S. Pat. No. 5,502,670 describes a method to flush all memory-cells of a volatile semiconductor memory with information of the same value, either all zeros or all ones. Nevertheless it has been determined that remnant charge after such an operation may still be sufficient to determine previous charge in each of the memory-cells, for example by the abovementioned electron microscopy, thereby corrupting security. It has also been determined that interrupting power supply to the volatile semiconductor memory and have stored charges flow away is an insufficient measure. It takes too long and even after old charges have flowed away it is still possible by, for example, electron microscopic probing to determine remnant charges in each of the memory-cells and thereby determine the original information contents of the memory.
The present invention overcomes the abovementioned limitations and provides method and device for making previous information contents of memory-cells of volatile semiconductor memory irretrievable.